1. Field of the Invention
The present invention relates to an X-ray computed tomography scanner and an X-ray-detection system, and more particularly, to an X-ray computed tomography scanner and an X-ray detection system, in which problems caused by noise or a fluctuation of an operating voltage in a detector or a DAS (data acquisition system) are solved thereby achieving a stable and reliable operation.
2. Description of the Related Art
An X-ray computed tomography scanner (hereinafter, referred to as an X-ray CT scanner) is known as one of computed tomography apparatuses. The X-ray CT scanner has been widely used for a long time as a tool of medical diagnosis and various medical research works. Since its advent, the X-ray CT scanner has progressed greatly and is still progressing.
In early types of X-ray CT scanners, it is capable of taking only a single slice of image at a time, and thus it is difficult to take many slices of images over a large range of a body being examined in a short time. Thus, in practical medical applications, there has been a strong need for a technique to take many slices of high-resolution images over a wide range of a body being examined in a short time.
To meet the above need, a multi-slice X-ray CT scanner has been developed recently and is becoming popular. In the multi-slice X-ray CT scanner, a two-dimensional detector array is used in which a specified number (for example, 4 or 8) of detector element columns each being similar to a linear array of detector elements used in a single-slice X-ray CT scanner are arranged in a direction perpendicular to the columns such that the two-dimensional detector array includes a total of M channels×N segments detector elements (where M and N are positive integers). A data acquisition system (DAS) is connected to outputs of the two-dimensional detector array.
The multi-slice X-ray CT scanner includes an X-ray source for emitting an X-ray beam spreading out in the form of a fan in a plane parallel to a slice direction and also includes a two-dimensional detector array of the type described above. Projection image data of a plurality of slices can be obtained at a time by detecting a cone-beam X-ray (with an effective diameter of field of view FOV) passing through a body being examined by the two-dimensional detector array. Thus, the multi-slice X-ray CT scanner is capable of acquiring a greater number of images with higher resolution than the single-slice X-ray CT scanner is capable of.
The two-dimensional detector array includes a scintillator, a photodiode array, and a CMOS switch array. An X-ray incident on the scintillator is converted into an optical signal, which is incident on the photodiode array and converted into an analog electric signal for each segment of the two-dimensional array. The electric signal of each segment is transferred to the DAS via a corresponding switch element of the CMOS switch array. Depending on a specified number of columns of detector elements in the slice direction, The CMOS switch array combines together electric signals output from elements of the photodiode array over each group including a specified number of columns of detector elements in the slice direction.
The DAS is a device including analog-to-digital converter elements (DAS circuit parts) arranged in the form of an array, for amplifying an analog detection signal received from the two-dimensional detector array and converting it into a digital signal. The resultant digital signal is subjected to an image reconstruction process.
Japanese Unexamined Patent Application Publication No. 2001-215281 discloses a detection system mainly including a two-dimensional detector array and a DAS that are similar to those described above and that are three-dimensionally disposed such that a photodiode array and a CMOS switch array are disposed on one surface of a double-sided circuit board and the DAS is disposed on the other surface. In this detection system, a scintillator block is disposed on the photodiode array such that they are optically coupled to each other. In practice, the above described parts are assembled on a block-by-block basis wherein each block is assigned a particular number of channels. In the typical X-ray CT scanner, the DAS usually has high circuit complexity to deal with as many channels as 500 to a few ten thousand channels. To reduce a difficulty with directly assembling such a large number of parts into a single piece, a circuit board is prepared for each block for handling several ten to several hundred channels, and a detector block is formed by disposing a photodiode array, a CMOS switch array, and a DAS part for dealing with assigned channels on either side of the circuit board. The plurality of detector blocks are disposed side by side on a partial surface of a sphere.
In this detection system, power is supplied to all detector blocks from a single power supply via a power cable.
The DAS deals with a small current. To avoid the small current dealt with by the DAS from being disturbed by noise intruding into the DAS via a ground path, a ground line of an electric circuit of the DAS is isolated from a base frame of the X-ray CT scanner, or, if the ground line is connected to the base frame, the connection is made only at a single point. In any case, a strong connection between the ground line of the electric circuit of the DAS and the base frame is avoided. The whole circuit of the DAS operates in synchronization with a single oscillator.
The conventional detection system described above has various unsolved problems.
A first unsolved problem relates to grouping of the DAS. Because the ground line of the electric circuit of the DAS is substantially isolated from the base frame, when the whole circuit of the DAS is operated in synchronization with the single oscillator, the ground level of the DAS has a fluctuation that causes noise to radiate from the ground line of the DAS and thus causes degradation in image quality.
Furthermore, the isolated ground causes a secondary problem that the fluctuation of the ground level of the DAS influences an operation of an analog signal.
Another problem is that a voltage drop occurs along the power cable via which the electric power is supplied to the detection system. The magnitude of the voltage drop varies in proportion to the length of the power cable, and thus the voltage drop varies from one detector block to another because the length of the power cable varies from one detector block to another. This produces non-uniformity of the operating characteristics of the DAS part among detector blocks. One possible technique to achieve sufficiently good uniformity of operation characteristics of the DAS part by reducing the difference in power supply voltage among detector blocks is to increase the diameter of the power cable extending from the power supply to each detector block as much as needed to reduce the voltage drop along the power cable to a sufficiently low level. Another technique is to set the length of each cable from the power supply to each detector block to be equal to the length of a longest cable from the power supply to a detector block at a most distant location. However, the both techniques cause an unacceptably large increase in space where the detection system is disposed, and thus both techniques cannot be realistic solutions.